The proposed research involves studying the molecular mechanisms involved in the regulation of the synthesis and secretion of thyrotropin (TSH). A pituitary tumor which is induced by radiothyroidectomy in mice is composed of a nearly homogeneous population of thyrotropes. In short-term cultures, cells derived from these tumors produce TSH; TSH production is stimulated by TRH and inhibited by thyroid hormones. In order to establish a more ideal cell system, we will attempt to adapt these cells to long-term culture and clone pure cell lines. We will define the kinetics of the synthesis and secretion of TSH and its alpha and beta subunits. The subcellular events involved in TRH stimulation will be characterized with respect to binding to putative receptors, to possible separate effects on TSH synthesis and secretion, to stimulation of synthesis of the subunits and a possible limiting role for the beta subunit, and to interactions with thyroid hormones. The subcellular events involved in thyroid hormone effect will be studied with respect to their binding to putative receptors, to effects on synthesis and secretion of TSH and its subunits, and to demonstration of a possible intracellular mediator of inhibition of TSH release.